Pentaerythritol phosphate is a white solid melting at 213.degree.-218.degree. C. It is characterized by the structure ##STR1## It is a useful intermediate in the preparation of flame-retardant materials, polyurethanes, and plasticizers.
It is well-known in the art that 3,9-dichloro-2,4,8-10-tetraoxa-3,9-diphosphaspiro [5,5]undecane-3,9-dioxide may be prepared ##STR2## by the reaction of pentaerythritol with two mols of phosphorus oxychloride. See U.S. Pat. No. 3,342,903 (Grabhofer et al.).
U.S. Pat. No. 3,155,703 (Emmons et al.) shows the reaction of trimethylol propane with phosphorous oxychloride, at room temperature, to form a tan semicrystalline mixture which upon crystallization from water yields 51% of a light tan crystalline precipitate. The product is ethyl bicyclic phosphate.
U.S. Pat. No. 3,189,633 (Chang et al.) shows the same reaction, but carried out in the presence of a substantial quantity of pyridine as a hydrogen chloride acceptor. Also, the use of a petroleum ether as a solvent is shown.
U.S. Pat. No. 3,134,662 (Gradsten) shows the reaction of trimethylol propane with thiophosphoryl chloride to produce trimethylolpropane thiophosphate. Benzene is employed as a solvent, the yield of a product is about 35-40%.
U.S. Pat. No. 3,287,448 (Ratz) teaches the preparation of 1-hydroxymethyl-4-phospha-3,5,8-trioxabicyclo [2,2,2]octane-4-sulfide, i.e., pentaerythritol and thiophosphoryl chloride. The reactants are mixed and heated at 160.degree.-170.degree. for six hours. A 63% yield of the product is obtained.
The oxidation of pentaerythritol phosphite to pentaerythritol phosphate is shown in U.S. Pat. No. 3,293,327 (Hechenbleikner et al.). The oxidation is accomplished with hydrogen peroxide, in isopropanol.
In all of these prior art processes, however, the yield of product is low, or it is necessary to use large amounts of a hydrogen-chloride acceptor (such as pyridine), or the product contains sulfur, or the process requires the availability of a relatively expensive starting material (pentaerythritol phosphite). None of these disadvantages inheres in the process of the present invention.